Super-radiant short pulse laser

ABSTRACT

Short intense lasing pulses of resonant radiation, of the order of 10 13 sec., are produced by employing adiabatic rapid passage of an optical transition of an optically resonant medium, such as K-vapor. The adiabatic rapid passage is achieved in the K-vapor by frequency modulating each pulse of a ruby laser giant pulse source. The modulated pulses are then passed through a fluoronitrobenzene Raman cell to provide an output pulse which has the required power and whose instantaneous carrier frequency sweeps through a resonance having a pair of levels connected by electric dipole transitions in the K-vapor system.

Lumpkin et al.

" l5] 3,657,554 Apr. 18, 1972 OTHER PUBLICATIONS Treacy et al., PhysicsLetters, June 16, 1969, pp. 369- 370.

Primary Examiner-Roy Lake Assistant Examiner-Darwin R. HostetterAttorney-Hanifin and Jancin and Joe L. Koerber [57] ABSTRACT Shortintense lasing pulses of resonant radiation, of the order of 10 sec.,are produced by employing adiabatic rapid passage of an opticaltransition of an optically resonant medium, such as K vapor. Theadiabatic rapid passage is achieved in the K-vapor b y frequencymodulating each pulse of a ruby laser giant pulse source. The modulatedpulses are then passed through a fluoro-nitrobenzene Raman cell toprovide an output pulse which has the required power and whoseinstantaneous carrier frequency sweeps through a resonance having a pairof levels connected by electric dipole transitions in the K- vaporsystem.

6 Claims, 3 Drawing Figures FILTER FLUORO- NITROBENZENE RAMAN CELL cusome PATENTEDAPR 18 1922 J o SWITCHED 7 POTASSIUM RUBY LASER FILTERVAPOR CELL L NbO L 3 FLUOR0-NITROBENZENE RAMAN CELL RUBY PULSE A L A r20 x10 10 ns RF E Y SINEWAVE 3 1342 (-50 mc/s) 3/2 0 10 x 10 II [LI 2 [UT ME F l G 3 INVENTORS OSCAR J. LUMPKIN NORMAN s. SHIREN ATTORNEYBACKGROUND OF THE INVENTION The present invention relates to asuper-radiant short pulse laser and more particularly to a super-radiantshort pulse laser initially pumped by adiabatic rapid passage. Inaccordance with the teachings of the present invention an arrangement isprovided for producing pulses which are both short in duration, in theorder of subpicosecond, and high in intensity.

The super-radiant state of a coherence brightened laser has beendescribed in the literature, as seen by reference to an article entitledThe Coherence Brightened Laser by R. H. Dicke in the Proceedings of the1963 Conference on Quantum Electronics, Vol. 1, page 35, Columbia Univ.Press, New York, 1964.

It is known in the prior art that conventional lasers employ mirrors inorder to produce feedback amplification and provide the conditionnecessary to effect coherent radiation. On the other hand, it is knownthat mirrors need not be used in a system employing a radiator which isinitially in a super-radiant state (as described by Dicke). In such anarrangement the directivity of the super-radiant transitions resultsfrom the pencil-like shape of the radiator wherein the radiator operatesin what is described as the end fire mode. The advantages of the latterlaser system reside in its simplicity, efiiciency and ability to producepulses of high intensity and short duration.

In a coherence brightened laser the radiation process can be describedin terms of a short and intense optical shock wave propagating along anelongated array of atoms. With all of the atoms in their excited statesa sufficiently intense pulse would dump the energy in each atom into thewavefront. Under such conditions the pulse would shorten because of thenon-linearity of the polarization response to a short transient excitingpulse. In such an arrangement, for the laser to be sufficientlybrightened to provide super-radiant emission virtually completepopulation inversion must be achieved wherein the condition to besatisfied is that where n is the number of atoms, all initially excited;c is the speed of light; w and L are the width and length of thepotassium vapor cell, respectively; A is the wavelength of the laser; Avis the incoherent line breadth including inhomogeneous broadening,doppler broadening and relaxation broadening in addition to radiationbroadening. Av, is the radiation line breadth.

SUMMARY OF THE INVENTION In accordance with the teachings of the presentinvention complete population inversion according to the above statedcondition is achieved by employing adiabatic rapid passage of an opticaltransition. For a general description of the basic principles ofadiabatic rapid passage reference is made to the text entitled ThePrinciples of Nuclear Magnetism by A. Abragam, pg. 34 et seq., 1961,Oxford Press, England. Adiabatic rapid passage, as will be described inaccordance with the principles of the preferred embodiment hereinaftershown, acts to provide the required means for elfecting a rapidinversion transition from the ground state to an excited state, for eachof the atoms in a two level electric dipole connected system, such thatthe total time for virtually complete population inversion is relativelysmall. The necessity of a small inversion time will become clear when itis recognized that to attain the super-radiant inversion conditionstated above, in addition to the requirement that the exciting pulse beof high intensity, it is also required that the exciting and emittedpulses traverse the medium in times short compared to the dominantrelaxation time for the system.

Alternatively, it may be said that super-radiance requires the excitingpulse to the system be of sufficient intensity to provide the requiredpopulation inversion and be of sufficiently short length such that theduration of the pulse is short as compared to the spontaneous relaxationtime whereby inversion is obtained in a time short compared to thespontaneous emission time.

Accordingly, it is an object of this invention to provide an improvedcoherent light pulse source.

It is a further object of this invention to provide a superradiant shortpulse laser.

It is yet another object of his invention to provide an arrangement forproducing lasing pulses of extremely short duration.

It is still another object of this invention to provide a laser devicewhich is simple and efiicient and which produces pulses of highintensity and short duration.

It is yet a further object of this invention to provide a coherencedbrightened laser for effecting super-radiant emission by adiabatic rapidpassage of an optical transition.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a preferred embodiment ofthe novel concepts in accordance with the principles of the presentinvention.

FIG. 2 shows the energy levels in potassium vapor through which opticaltransition is obtained by adiabatic rapid passage in accordance with thepreferred embodiment.

FIG. 3 shows the time relationship of the ruby laser pumping pulse tothe RF signal modulating the pulse carrier frequency of the lasersignal.

A DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown apreferred arrangement for producing super-radiant laser pulses byadiabatic rapid passage of an optical transition in a two-levelpotassium vapor system. However, it is clear that within the teachingsof the present invention adiabatic rapid passage to effect super-radiantlaser pulses may be achieved in a medium other than potassium vapor.Likewise, it is clear that although a two-level system within potassiumvapor is described, other than twolevel systems may be employed inaccordance with the teachings of the present invention.

To achieve adiabatic rapid passage of an optical transition to effect apopulation inversion for coherence brightening in a medium, it is firstrequired that the medium be subjected to a strong light pulse thecarrier frequency of which is swept, from either above or below, throughthe resonant frequencies of a I transition resonance of the medium. Itis also required that the rate at which the carrier frequency is sweptbe small compared to the intensity of the pumping light pulse such thatwhere Aw is the width of the resonance line and T is the relaxation timefrom the excited state to the ground state.

In the preferred embodiment of FIG. 1 there is shown an arrangement forproducing adiabatic rapid inversion for the 4 8 4 P transition inpotassium vapor. A high intensity light pulse source 1 is shown, whichlight pulse source is, in the preferred embodiment, a Q-switched rubylaser of conventional design. The output pulses of the Q-switched rubylaser are polarized parallel to the C axis of LiNbO electro-opticaldoppler shifter 3. The output of RF signal source 5 is applied to theLiNbO body 7 to modulate the carrier frequency of the ruby pulsesreceived from Q-switched ruby laser 1. For a more complete discussion offrequency modulation of laser pulses as hereinabove described referenceis made to an article entitled Compression of Optical Pulses byGiordmaine et a]. in the IEEE Journal of Quantum Electronics, Vol. QE-4, No.5, May 1968.

The frequency modulated output of LiNbO electro-optical doppler shifter3 is applied to excite fluoro-nitrobenzene Raman cell 9, which cell isknown to those skilled in the art. The output pulse fromfluoro-nitrobenzene Raman cell 9 is then passed through optical filter11, to remove incident ruby light, and thereafter applied to excitepencil-like potassium vapor cell 13, to effect adiabatic rapid passageof the 4 8, 4 P transition therein. Super-radiant pulses are emitted at1 It should be noted that although a pencil-shape vapor cell has beendescribed and shown in the preferred embodiment, other shapes may alsobe employed. Although super-radiant emission is not generally criticallydependent upon the shape of the vapor cell employed, the emittedradiation pulse width and frequency spread does depend upon shape, aswell as the density of the vapor atoms. Thus, cell shapes and atomdensity may be selected in accordance with desired duration of theemitted super-radiant output pulse.

It is also significant to note that the Stokes output of afluoro-nitrobenzene Raman cell, excited by a ruby laser giant pulse, isresonant with the potassium vapor 4 S, 4 P transition and is ofsufficient power, i.e., 300 KW (peak power) at about 13,044 cm, toeffect the transition. Such has been described by O. J. Lumpkin, Jr. inan article entitled Four-Wave Parametric Interaction in Potassium Vaporin the IEEE Journal of Quantum Electronics, April 1968, Vol. QE-4, No.4.

Thus, the fluoro-nitrobenzene Raman cell acts to bring the ruby lasergiant pulse frequency into the 4 S, 4 P transition frequency rangewherein the RF source 5 of frequency modulator 3 may sweep the rubylaser giant pulse from above the said transition frequency range throughresonance. FIG. 2 shows the 48 and 4P potassium vapor energy levelswherein pumping action about the 4P level effects an energy leveltransition from the 45 level as indicated by the arrow.

In accordance with the conditions above stated, the pumping pulse source1 and RF source 5, in FIG. 1, are chosen so as to effect a frequencysweep of the ruby laser giant pulse carrier frequency at a rate which issmall compared to the fluoronitrobenzene Raman cell intensity but whichfully sweeps a giant pulse of sufficiently short duration so as toeffect a sweep through the potassium transition resonance line in a timesmall compared to the latters relaxation time. In this respect a rubylaser giant pulse of approximately 10 ns modulated by a sine wave ofaround 50 mc/s has been found adequate and is given by way of exemplaryvalues. FIG. 3 shows the optimum timing relationship of the ruby pulseto RF modulating electric field to efiect adiabatic rapid passagemodulation for the coherence brightening and super-radiant emission inpotassium vapor in accordance with the principles of the presentinvention.

What is claimed is:

1. A coherence brightened short pulse laser wherein the necessary statefor coherence brightened emission is obtained by adiabatic rapid passageof an optical transition in the lasing medium, comprising:

a laser cell means containing vapor asthelasing medium;

a pulsed pumping source means for producing a coherent light pulse ofhigh intensity for excitation of said vapor in accordance with therelationship 12 25f fir.

and

means for sweeping the instantaneous frequency of said n cw Av N cAuAZLAYMLLQIEQL 2. The laser as set forth in claim 1 wherein saidinversion is between a pair of levels connected by an electric dipoletransitron.

3. The system as set forth in claim 2 wherein said vapor is potassiumager,

fTlTesystem as set forth in claim 3 wherein the said means for sweepingfurther comprises:

RF modulation means for frequency modulating said soherqn ightnu se;

a fluoro-nitfobenzene Raman cell for bringing the frequency range of thefrequency modulated coherent light pulse into the 4 8 4 P transitionfrequency range.

5. The system as set forth in claim 4 wherein said pulsed pumping sourcemeans includes a Q-switched ruby laser.

6. The apparatus as set forth in claim 5 wherein said RF modulatingmeans includes a LiNbO medium for receiving said light pulses to bemodulated therein by an RF electric field created by an RF modulatingsignal means.

1. A coherence brightened short pulse laser wherein the necessary state for coherence brightened emission is obtained by adiabatic rapid passage of an optical transition in the lasing medium, comprising: a laser cell means containing vapor as the lasing medium; a pulsed pumping source means for producing a coherent light pulse of high intensity for excitation of said vapor in accordance with the relationship and means for sweeping the instantaneous frequency of said light pulse through the resonant frequency of an optically resonant transition of said vapor at a rate sufficiently rapid in accordance with the relationship for producing adiabatic population inversion and coherenced brightening thereby effecting super-radiant emission by satisfying the relationship
 2. The laser as set forth in claim 1 wherein said inversion is between a pair of levels connected by an electric dipole transition.
 3. The system as set forth in claim 2 wherein said vapor is potassium vapor.
 4. The system as set forth in claim 3 wherein the said means for sweeping further comprises: RF modulation means for frequency modulating said coherent light pulse; a fluoro-nitrobenzene Raman cell for bringing the frequency range of the frequency modulated coherent light pulse into the 42S1/2 -> 42P3/2 transition frequency range.
 5. The system as set forth in claim 4 wherein said pulsed pumping source means includes a Q-switched ruby laser.
 6. The apparatus as set forth in claim 5 wherein said RF modulating means includes a LiNbO3 medium for receiving said light pulses to be modulated therein by an RF electric field created by an RF modulating signal means. 